Container refrigeration apparatuses have been used to cool the interior of a container for use in, e.g., marine transportation (see, e.g., Patent Document 1).
The container is loaded with plants such as bananas and avocados. Plants perform respiration by absorbing oxygen in the air and releasing carbon dioxide even after they are harvested. If the oxygen concentration in the container is reduced to a predetermined target concentration as a result of the plant respiration, the respiration rate of the plant decreases. However, since it takes more time to reach such a target concentration, the plants will be discolored, rotten, or deteriorated in other forms in the meantime to have a decreased degree of freshness.
Patent Document 1 discloses a configuration in which the oxygen concentration in a container is quickly reduced by separating nitrogen from the air with a membrane separator to produce nitrogen gas, and supplying such nitrogen gas into the container. As can be seen, if the oxygen concentration of the air in the container is set to be lower than that of the outside air, the respiration rate of the plants may be reduced so much that the degree of freshness can be maintained more easily.
In order to keep the plants fresh, however, it is necessary to properly control not only the oxygen concentration, but also a carbon dioxide concentration, in the container. For example, when the plants are bananas, it is recommended to maintain a nitrogen concentration of 5% and a carbon dioxide concentration of 5% in the container.
Here, in the container, carbon dioxide is generally released as a result of the plant respiration to increase the carbon dioxide concentration in the container. However, the apparatus disclosed in Patent Document 1 supplies the interior of the container with a gas mixture containing 99% of nitrogen and 1% of oxygen. Thus, the plants cannot respire sufficiently, and the carbon dioxide concentration remains low in the container.
Thus, in the apparatus disclosed in Patent Document 1, carbon dioxide bottles are separately provided to supply the interior of a container with carbon dioxide, and control the carbon dioxide concentration in the container.